The invention relates to a coated bipolar plate for electrochemical energy converters, such as is typically used for fuel cells and/or electrolysis cells. The invention furthermore relates to a method for producing the coated bipolar plate according to the invention.
An electrochemical energy converter (such as a fuel cell or an electrolytic cell) comprises two electrodes (anode and cathode), which by means of a solid semipermeable electrolyte membrane or a liquid electrolyte in an inert fiber structure are disposed in an electrically insulated and mechanically separated manner and form what is known as a membrane electrode assembly (MEA). In fuel cells, the oxidation reaction takes place at the anode and the reduction reaction takes place at the cathode, the opposite being true for electrolyzers. Depending on the cell type, ion-conducting polymer membranes, solute lyes or acids, alkali carbonate melts or ceramics may serve as the electrolyte. A porous electrically conductive gas diffusion layer made of carbon non-woven/woven fabric or metal foam abuts the electrode on both sides. Adjoining the same, an electrochemical energy converter comprises a bipolar plate (also referred to as flow distributor or current collector) on both sides, which are usually made of electrically conductive carbon composite materials or metals.
The focus is on fuel cells or electrolyzers that are operated by way of a solid polymer electrolyte membrane. A distinction is made between low-temperature polymer electrolyte fuel cells/electrolytic cells, which are operated at approximately 85° C., and high-temperature polymer electrolyte fuel cells/electrolytic cells, which operate in the temperature range between 120° C. and 180° C. In both variants, a proton-conducting or hydroxide ion-conducting ionomer membrane serves as the electrolyte. This is gas-tight and not electron-conducting. Adjoining the same, a catalyst layer (electrode), a porous gas diffusion layer and a bipolar plate are each provided on both sides. On the electrode surface, three phases (a catalyst serving as the electron conductor, an ionomer serving as the proton conductor and a reactant) are always in contact. The reactants (which are hydrogen and oxygen in the fuel cell and water in electrolytic cells) are fed via the flow field of the bipolar plates and homogeneously distributed across the catalyst surface by way of the porous gas diffusion layer. The role of the bipolar plate is to mechanically stabilize the electrochemical cell, supply and remove reactants on both sides, and withdraw the generated electrical current.
Bipolar plates must therefore be mechanically stable because, as they serve as the mechanically supporting element inside fuel cells and electrolyzers, they must withstand thermal expansion, high pressing pressure and, in mobile applications, vibrations and oscillations. Bipolar plates must also have high electrical and thermal conductivity so as to efficiently withdraw the generated electrical current and give off thermal energy to the cooling medium. For this purpose, they must have dense surface properties, so as not to take up any reactants or electrolyte and so as to separate these spatially from the cooling medium. Moreover, they regulate the water content (supply of water in the case of electrolyzers, and discharge of water in the case of fuel cells). Additionally, high stability with respect to electrochemical corrosion at high temperatures and an external potential is required. Among other things, the bipolar plates must withstand concentrated phosphoric acid at temperatures up to 180° C. and electrochemical potentials up to approximately 2.2 V (vs. reversible hydrogen electrode). The electrochemical corrosion at the bipolar plate/electrolyte phase boundary represents the decisive criterion in the material selection for bipolar plates.
At present, the use of graphite-polymer composites represents the state of the art, both for low-temperature and for high-temperature applications. These are organic polymers, such as polypropylene, polyphenylene sulfide, phenolic and vinyl ester resins comprising carbon black or graphite particles, which exhibit a considerable improvement in robustness and in mass production by way of injection molding processes or hot pressing, compared to pure graphitic materials.
While the polymer matrix of such graphite-polymer composites improves the mechanical stability due to elastic properties, this comes at the expense of electronic and thermal conductivity, since the polymer fractions have an insulating effect. Moreover, graphite-based bipolar plates have a relatively high material thickness (>2 mm).
This is where the enormous advantages of metallic bipolar plates come in. In general, these can be generated using simple and inexpensive production methods, such as stamping or high-pressure forming. Furthermore, they exhibit good ductility and considerably higher mechanical stability against impact and vibrations, which frequently present the problems of causing cracks and, as a consequence, loss of gases. Moreover, they have sufficiently high electronic and thermal conductivity for use in a fuel cell or in an electrolyzer.
One significant challenge of metallic bipolar plates, for which the possibility of a broad introduction into fuel cell technology had previously been denied, is the susceptibility of the metallic materials used in bipolar plates to corrosion in a moist acid/alkaline environment.
While moderate corrosion conditions (electrolyte concentration <0.5 M H2SO4 at ˜85° C.) are present in low-temperature fuel cells/electrolyzers, more drastic conditions prevail in high-temperature fuel cells/electrolyzers due to the concentrated phosphoric acid (up to 16 M) used and the high temperature of up to 180° C. This causes the metallic bipolar plates to corrode upon contact with the electrolyte, and the released metal ions can disadvantageously poison the polymer electrolyte membrane (Nafion® or polybenzimidazole membrane). The decrease in proton conductivity as a result of the intercalation of metal ions has already been examined in the low-temperature application using Nafion® membranes. Another occurrence of corrosion is the formation of non-conducting or insufficiently conductive passivation layers (metal oxides, metal hydroxides, metal phosphates) on the metal surface of a bipolar plate, which are accompanied by a rise in the electrical contact resistance. This has also already been examined in-depth for sulfuric acid conditions (Nafion® membrane). In addition to the high operating temperature, the electrochemical potential is also a decisive factor influencing corrosion. Potentials of approximately 1 V can occur in fuel cells, and in electrolyzers they can even be as high as 2.2 V (vs. reversible hydrogen electrode).
The aforementioned disadvantages of corrosion can be reduced by applying resistant and electronically conductive coatings to the metallic bipolar plate. Coatings known thus far are, for example, ceramic nitrides and carbides based on titanium, chromium, aluminum, silicon or zirconium, graphitic or gold-based coatings, produced by way of physical or chemical vapor deposition (PVD/CVD). From the literature, electrochemical deposition processes of metal borides (such as NiCoB, Ni2B or Ni3B), gold or conductive organic polymers, such as polyaniline or polypyrrole, are also known. The constant difficulty encountered with all coating options is to produce defect-free layers that offer high long-term stability. Even minute defects such as cracks or pinholes cause the electrolyte to spread beneath the electrolyte and result in corrosion damage.
Another option is to plate stainless steel, serving as the bipolar plate material, with a thin niobium layer, which forms stable and electronically conducting oxide layers and thereby passivates the metal.
In principle, not only the chemical and electrochemical stability, but also the electrical conductivity and the thermal coefficient of expansion between the metallic bipolar plate material and the applied coating material must be considered in the selection of coatings, so as to avoid surface cracks.
Bipolar plates in polymer electrolyte fuel cells are subject to electrochemical corrosion due to the acid environment and the influence of the temperature and electrochemical potential. Only precious metals, such as gold, or very rare and expensive metals, such as tantalum, which forms a stable and conductive oxide layer under the above-described conditions, can be used for conditions this drastic. Inexpensive stainless steels and nickel base alloys, in contrast, have excessively high corrosion rates and/or form non-conductive passive layers. This results in a relatively rapid decrease in performance and the disadvantage of aging of the fuel cell.
So as to still be able to utilize the decisive advantages of metallic bipolar plates, a cost-effective corrosion-resistant and electronically conductive coating and a coating method suitable for mass production are needed.